Transparent image like a decal on mesh with principled BSDF shader
I find this questions easier to understand if you don't think in terms of transparency and think in terms of Masks to control how elements mix.

Using the Alpha channel in a mix node will make White areas use one element, and Black areas use the other. Any shade of gray will be a partial mix according to its brightness.



What that means is that the alpha channel of a given image can be used as a Mask to control the mix of color or the mix of shaders. 

When you use one or the other is something you have to decide.


If you want to use the color information of the image over the color of a given shader, in a way that all other properties of the shader are homogeneous, then use a color mix node and have the mask be the factor for the color of the shader.




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Or you can use the mask to control the mix between two different shaders. Note that doing this will make the masked areas have different characteristics.




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Lastly if what you really want is the mask to control the transparency of the object, then use the alpha information as the factor that controls the mix of a transparent and other kind of shader.




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